I found already to connect the green wire in 20 pin ATX to a ground (black wire) and the PSU starts. But as soon as I connect a harddisk it stops.
What else should i do to keep the PSU running while several Harddisks are connected.
Thanks for the update, yeah, sometimes when a PSU is overloaded it trips the overcurrent protection which shuts itself down to prevent a fire. When they go bad, sometimes it false trips at low loads and makes it look like something else is going on.